In order to cool an automobile engine, generally a water-cooling type cooling system is employed using a radiator. In conventional types of such cooling systems, a thermostat employing a thermally expanding body, or a valve unit under electric control, has been used to adjust the amount of cooling water circulated to the radiator side, in order to enable control of the temperature of the cooling water introduced to the engine.
That is, a thermostat using the above thermally expanding body or a control valve employing a valve unit under electrical control or similar intervenes in a portion of the cooling water path, such as for example on the entrance side or on the exit side of the engine, so that when the cooling water temperature is low, the control valve is closed and the cooling water is circulated through a bypass path without passing through the radiator, and when the cooling water temperature is high, the control valve is opened and the cooling water is circulated through the radiator, so that the temperature of the cooling water can be controlled in a desired state.
For example, the above thermostat may comprise a sensor case incorporating a thermally expanding body, in proximity to the circulating flow path of the cooling system, which detects changes in the temperature of the cooling water to expand and shrink; [the thermostat] has a function to open and close a value through changes in volume accompanying the expanding and shrinking of this thermally expanding body, so that the cooling water is maintained at a prescribed temperature.
An example of a conventional thermostat device appears in FIG. 8. Explained in simple terms, this thermostat device 1A comprises a thermostat actuation portion 30, which is the valve driving portion, within a valve housing 31, 32 comprising two members. This thermostat actuation portion 30 is fixed to the valve housing 32, and causes the first valve body 33 and second valve body 35 (bypass valve) to act through an actuating piston (not shown) mounted within the thermostat actuation portion 30, exercising control to selectively link and block the main path 34 and bypass path 36.
The housing 37 of the thermostat actuation portion 30 is held by a holder 39; this holder 39 surrounds, in cap shape, the end portion [of the housing 37] on the side opposite the side from which the above actuating piston in the housing 37 protrudes. The cap-shape holder 39 is supported by the valve housing 32.
A pot-shape central region is provided in the profile of the first valve body 33, and the thermostat actuation portion 30 is positioned in the center portion thereof. When the first valve body 33 is in the closed position, this central region is in a state which blocks the holder 39 and the main path 34. A bolt 38 is provided in a line extended in the axial direction of the actuating piston, not shown, of the thermostat actuation portion 30; on this bolt 38 is mounted a second valve body 35 (bypass valve) by means of a sliding guide, to open and close a bypass path 36. The second valve body 35 (bypass valve) is impelled by a spring member 41. The first valve body 33 is impelled by the spring member 40, and this spring member 40 is supported by the valve housing 31.
When for example a thermostat device 1A with such a configuration is positioned as an entrance control, cooling water flowing in from the radiator exit side through the sleeve 42 passes through the main path 34 and is supplied in circulation to the engine entrance side from the sleeve 43, or, cooling water from the engine exit side flows in from the bypass path 36, and is supplied in circulation to the engine entrance side from the sleeve 43.
Prior to warmup at the time the engine is started, the cooling water flowing in from the radiator exit is at a low temperature, so that the wax within the housing 37 is in a shrunken state, the first valve body 33 is in a state of blocking the main path 34, and the second valve body 35 (bypass valve) leaves open the bypass path 36. Consequently cooling water flowing in from the bypass path 36 is returned directly to the engine entrance side via the sleeve 43.
After warmup operation, when the cooling water reaches the prescribed temperature, the wax within the housing 37 expands due to this temperature, to extend the actuating piston, not shown, of the thermostat actuation portion 30. Through extension of this actuating piston, the first valve body 33 moves downward against the impelling force of the spring member 40 to change from a state of blocking to a state of opening the main path 34, and in addition the second valve body 35 (bypass valve) blocks the bypass path 36. Hence cooling water which had flowed in from the sleeve 42 passes through the sleeve 43 to flow into the engine entrance portion.
In order to forcibly expand the wax within the housing 37, a construction may be employed in which a heated means 44 is provided within the housing 37, and this heated means 44 is caused to protrude from the valve housing 32 and to make contact with a heating element 45.
In such a conventional thermostat device 1A, the housing 37 which is the cooling water temperature-sensing portion is positioned inside the cooling water path, so that the resistance to flow of cooling water flowing in from the sleeve 42 is increased, and consequently it has been difficult to reduce the size of the water pump which forcibly circulates the cooling water.
Further, during warmup operation at the time the engine is started, cooling water from the engine exit side and cooling water from the radiator exit portion become mixed near the housing 37, so that a hunting phenomenon may occur; when such a phenomenon occurs, the temperature of cooling water flowing into the engine entrance side is unstable, and it becomes difficult to improve fuel efficiency and to quickly warm cooling water. As a means of preventing this [phenomenon], innovations must be made in the construction of the valve housing 32 for mixing of cooling water immediately before the housing 37 which is the temperature-sensing portion, and a current plate, called a baffle, must be installed.
Further, when the thermostat device 1A is positioned for entrance control, cooling water flowing in from the engine may also be detected, so that overshoot and other erroneous operation may occur.
In order to resolve such problems, the applicantsii previously devised [the invention of] Japanese Patent Laid-open No. 2001-317355 and similar; this preceding example has a construction in which a thermoelement temperature-sensing portion to actuate a main shaft controlling the opening and closing of a first valve body and a second valve body is positioned in contact only with cooling water from the engine exit side to sense temperature, without making direct contact with the cooling water from the radiator exit side; Further, a heat-generating element is mounted in the temperature-sensing portion, and the opening and closing of the first valve body and second valve body are controlled through control of heat generation by this heat-generating element as well.
When such a configuration is adopted, because the thermo-element is not positioned within the housing, resistance to the flow of cooling water within the housing can be reduced; and there are also such advantages as structural improvement of the valve housing for mixing of cooling water immediately before the housing and elimination of the need to install a current plate.
In such a thermostat device, through configuration as an electronically controlled thermostat in which, by controlling actuation of the above heat-generating element according to the various running conditions of the automobile, the engine cooling water temperature can be arbitrarily variably controlled, there is the following problem.
To explain this problem, in the above-described preceding device example the thermo-element is covered with a resin member, relying on the thermal insulation effect in order that there is no influence by the temperature of cooling water from the radiator, which may impact the performance of variable control of the cooling water temperature under electronic control. But in such a configuration, even when the cooling water temperature on the radiator exit side is extremely low, the valve may remain open without sensing this [temperature], so that more cold cooling water than necessary is supplied to the engine, raising concerns of damage to the engine and possible mission failure.
In order to eliminate such problems, a temperature sensor capable of detecting the temperature of cooling water from the radiator exit side may be provided, and electronic control performed with the detected value as a parameter; but this results in an increased number of components and higher costs.
The present invention was devised in light of these circumstances, and has as an object the obtaining of an electronically controlled thermostat enabling prompt warming of cooling water even at the time the engine is started, enabling improved fuel efficiency and reduced resistance to the flow of cooling water, in addition to appropriate and reliable operation according to operating conditions, including the temperature of cooling water from the radiator exit side, and which is simple in construction and inexpensive.